Why is stainless steel passivated

As a general rule, the higher the nickel and chromium content in the alloy, the more corrosion resistance it will have. In addition, APT can meet the requirements of any company-specific nitric or citric acid stainless steel passivation methods.

Our company also provides passivation of stainless steel to the Carpenter A-A-A process to passivate difficult stainless steel alloys including high-sulfur free machining or alloys as well as high chromium alloys such as stainless reference: A-A-A Passivation Methods article in our Metal Finishing White Papers section. Inhibited passivation solutions are available to maintain bright surfaces of stainless components such as machined faces and centerless ground or stainless steel shafts.

Unfortunately, very few part prints indicate the specific method to be employed. Advanced Plating Technologies provides full in-house testing services to certify the performance of our stainless steel passivation including high humidity, salt spray per ASTM B, potassium ferricyanide.

Citric 5: Other combinations of temperature time and concentration of citric acid with or without chemicals to enhance cleaning, accelerants or inhibitors capable of producing parts that pass the specified test requirements.

Immersion bath to be controlled at pH of 1. Avoid mixing grades of stainless steel e. In this situation, the less noble metal corrodes faster than it would have if the dissimilar metals had not been in contact in the solution. Best Technology is recognized as an industry leader in passivation equipment , tanks, systems and lines.

Our experts understand the careful balance of chemistry, temperature and immersion time to meet specifications and to avoid costly errors. We offer a broad range of equipment from tabletop machines to integrated wet benches to fully automated systems. Our application engineers can design equipment to meet your requirements and specifications. As you gather information on starting a new passivation line, be sure to check out our passivation process checklist.

Passivation equipment is available in a variety of tank sizes. The smallest systems start with a tank size of 1. A passivation system offers the integrated convenience of facilitating multiple process steps e.

Small Benchtop Passivation Equipment. Wet Bench Passivation Equipment. Automated Passivation Systems. Agitated Immersion Passivation Systems. In the aerospace and medical device industries, many high-precision manufacturers face additional guidelines, specifications, regulations and accreditation standards when passivating their products.

Use of an automated passivation system ensures tight, documented process control parameters to meet validation requirements. Within the medical device world there is a need to validate the passivation process. But what does that mean, and how does that work? Validation is the process of insuring that the passivation process you use will reproduce repeatable and predictable results every time a batch of parts is run through the process. By validating the process you are able to forego subjecting every part to testing to prove that it is properly passivated.

The IQ or Installation Qualification is the first part. It is developed by describing the machine — what is it?

It also looks at what the components on the machine are, gauges, switches, PLC, etc. It provides a description of the machine and its parts — what is it and how does it work? The OQ or Operational Qualification is the second part.

It essentially help you verify the IQ — does the machine operate as it is supposed to? Is that like plating, paint or what? What colour is it? How much tolerance should I allow for it? What is the point of using stainless steel if you are going to put some kind of coating on it anyway? Joe is not the exception. Many machine shops, purchasing agents and engineers are somewhat in the dark when it comes to the relationship between corrosion resistant stainless steel and chemical passivation.

Even among the finishing community, there is some disagreement about the theory behind the process of chemical passivation. Some believe it is effective because it is a cleaning process. Others credit the enhanced corrosion resistance properties to the thin, transparent oxide film resulting from chemical passivation.

Regardless, the bottom line is that it works. Verification tests, including copper sulfate immersion, and accelerated corrosion tests, such as salt spray, high humidity and water immersion, undisputedly confirm the effectiveness of chemical passivation.

Advanced material engineers in aerospace, electronics, medical and similar high-tech industries have used chemical passivation for years. The applications demand the maximum performance from components manufactured from corrosion-resistant steels, and they realize that passivation is one of the most effective methods of achieving these results.

These contaminants are potential corrosion sites that result in premature corrosion and ultimately result in deterioration of the component if not removed. In addition, the passivation process facilitates the formation of a thin, transparent oxide film that protects the stainless steel from selective oxidation corrosion. So what is passivation? Is it cleaning? Is it a protective coating? It is a combination of both. The process typically begins with a thorough cleaning cycle.

It removes oils, greases, forming compounds, lubricants, coolants, cutting fluids and other undesirable organic and metallic residue left behind because of fabrication and machining processes. General degreasing and cleaning can be accomplished many ways, including vapor degreasing, solvent cleaning and alkaline soaking. That is because the sulfides of sulfur-containing free-machining grades are partially or totally removed during passivation in a typical nitric acid bath, creating microscopic discontinuities in the surface of the machined part.

Even normally efficient water rinses can leave residual acid trapped in these discontinuities after passivation. This acid can then attack the surface of the part unless it is neutralized or removed. To effectively passivate the free-machining stainless steels, Carpenter has developed the A-A-A alkaline-acid-alkaline process that neutralizes trapped acid.

This method of passivation can be accomplished in less than 2 hours. Here is the step-by-step procedure:. Then rinse the part thoroughly in water.

After removing the part from this bath, flush it with water, then immerse it in the sodium hydroxide solution for another 30 minutes. Rinse the part again with water and dry it, completing the A-A-A method.

The benefits of this method are shown in Figure 3. Citric acid passivation has become increasingly popular with manufacturers who want to avoid the use of mineral acids or solutions containing sodium dichromate, along with the disposal problems and greater safety concerns associated with their use.

Citric acid is considered environmentally friendly in every respect. Although citric acid passivation offers attractive environmental advantages, shops having success with mineral acid passivation and suffering no safety issues might want to stay the course. There may be no real need to change if those users have a clean shop, well-maintained and clean equipment, coolant free of iron-containing shop dirt, and a process that yields good results.

Passivation treatment in citric acid baths has been found useful for a large number of stainless steel families, including several individual stainless grades, as summarized in Figure 4. The conventional nitric acid passivation methods from Figure 2 are included for convenience. Note that the older nitric acid formulations are in volume percent, while newer citric acid concentrations are in weight percent. The passivation treatment varies depending on chrome content and machinability characteristics of the grades in each family.

Note the columns referring to Process 1 or Process 2. As shown in Figure 5, Process 1 involves fewer steps than Process 2. Factors causing this attack included excessive bath temperature, excessive immersion time and bath contamination. The ultimate choice of passivation method will depend on the acceptance criteria imposed by your customer. Tests are often performed to evaluate the surface of passivated parts.

It is important that the test method be matched to the grade under evaluation. A test that is too severe will fail perfectly good material, while one that is too lenient will allow unsatisfactory parts to pass.

The cross section is usually the most critical surface, particularly for free-machining grades. One reason for this is that the sulfides, elongated in the direction of working, intersect this surface. Critical surfaces should be positioned upward, but at 15 to 20 degrees from the vertical to allow any moisture to run off. Material that has been properly passivated will be virtually free of rust, although it may show some light staining. Austenitic non-free-machining stainless grades also may be evaluated by means of a humidity test.

When so tested, liquid droplets of water should be present on the surface of samples, revealing free iron by the presence of any rust formation. As an alternative, the part may be immersed in the solution for 6 minutes.